Metallic surface coatings are commonly applied to electronic devices and decorative objects to provide corrosion protection and other desired functional properties. Electronic devices comprising copper or copper alloys typically comprise metallic surface coatings which provide corrosion protection, high surface contact resistance, and wear resistance. To meet these requirements, the connectors industry has developed surface coatings comprising one or more metallic layers. For example, a metallic surface coating can comprise a base metal underlayer and a precious metal overlayer. The base metal underlayer, such as a nickel underlayer, is coated over the copper or copper alloy substrate. The base metal serves as a diffusion barrier. The precious metal overlayer, such as gold, palladium, or alloys thereof, is then coated over the base metal underlayer coating. The precious metal overlayer provides corrosion resistance, wear resistance, and high conductivity. In a known metallic surface coating, a nickel underlayer increases the hardness of a gold overlayer. This metallic surface is commonly referred to as “nickel-hardened gold” or simply, “hard gold.” Variations on these coatings involve base metal alloy underlayers, precious metal alloy overlayers, and metallic surface coatings comprising two or more base metal underlayers and/or two or more precious metal overlayers.
An obvious disadvantage to the use of precious metals such as gold and palladium is cost. A cost effective connector uses a precious metal coating layer which is as thin as possible, without sacrificing the desired functional properties. Accordingly, the industry typically employs precious metal layer on the order of about 1.0 μm thick on electronic connectors. Thinner layers suffer from the disadvantage of highly increased porosity in the coating. Over time in service, the thin layers having a high degree of porosity are ineffective against base metal and copper diffusion to the surface. In a corrosive environment, the exposed base metal and copper will corrode and the corrosion product(s) can migrate onto the coating surface and deteriorate the surface contact conductivity. Moreover, a thin precious metal layer can wear off during application and shorten the connector's useful lifetime.
For many years, bare boards comprising copper circuitry were finished with eutectic tin-lead solder coating according to the Hot Air Solder Leveling (HASL) process. Due to the Restriction of Hazardous Substances (ROHS) directive, the industry has moved away from using lead as a component of the final finish of bare boards. One alternative final is electroless nickel-immersion gold (ENIG). ENIG is vulnerable to common pollutants and is sensitive to high humidity and tends to fail due to corrosion.
In a reported treatment of nickel-hardened gold surface coatings over copper alloy devices, alkyl phosphonates were applied to the metallic surface coating to block pores in the gold overlayer. See U.S. Pat. No. 5,853,797. The alkyl phosphonates chemically bonded to exposed nickel (through pores in the gold overlayer) and thereby inhibited nickel corrosion.
In a reported treatment of copper alloys to prevent corrosion, aromatic heterocycles comprising nitrogen such as imidazole and benzimidazole were applied to a copper alloy surface to inhibit copper oxidation and preserve solderability of the copper alloy surface. See U.S. Pat. Nos. 4,395,294 and 5,795,409.
A need continues to exist in the connectors industry for a metallic coating surface which uses as little precious metal as possible while still retaining advantages the precious metal overlayer provides. Moreover, a need continues to exist for an ENIG finish over copper circuitry in PCB manufacture that is less vulnerable to corrosion.